Electrode for fluorescent lamp



z- 26, 1958 H. WEISS 2,849,637

ELECTRODE FOR FLUORESCENT LAMP Filed Feb. 2, 1956 IN V EN TOR. HARRYWEISS AT TORNEYS United States Patent ELECTRODE FOR FLUORESCENT LAMPHarry Weiss, Larchmont, N. Y. Application February 2, 1956, Serial No.563,018

8 Claims. (Cl. 313-109) This invention relates to fluorescent lamps, andmore particularly to the electrodes thereof.

Some fluorescent lamps of the hot cathode type require preheating, andtherefore require a starter. More recent fluorescent lamps are madewithout a starter, these being hot cathode instant start lamps,frequently referred to as slimline lamps. The electrode is a wirefilament, usually tungsten, coated with an electron emissive material,usually alkaline earth oxides. The filament may be a coiledcoil, or acoiled-coil-coil.

During operation of a lamp some emission from the electrode may sputteron to the glass envelope at each end, causing a darkening which isunsightly and reduces illumination. Finally the lamp may fail. Thebelief (probably erroneous) up to now was that use of a great thicknessor filling of the alkaline earth oxide would increase the life of thelamp.

The primary object of the present invention is to prolong the life offluorescent lamps. Lamp life is dependent largely on the number ofstarts, rather than on the total number of operating hours, andaccordingly another object of the present invention is to increase thenumber of starts of a fluorescent lamp before darkening appears. Furtherobjects are to generally improve fluorescent lamps and their electrodes.

To accomplish the foregoing general objects, and other more specificobjects which will hereinafter appear, my invention resides in thefluorescent lamp and electrode elements, and their relation one toanother, as are hereinafter more particularly described in the followingspecification. The specification is accompanied by a drawing, in which:

Fig. 1 is a front elevation of a slimline fluorescent lamp.

Fig. 2 shows an electrode prior to coating the same with electronemissive material;

Fig. 3 is a section through a fragment of the electrode while still infilamentary form, with the upper half in section and the lower half inelevation;

Fig. 4 is a similar fragmentary section after a first coating withcolloidal graphite;

Fig. 5 is a similar fragmentary section after a second coating ofelectron emissive material;

Fig. 6 is a fragmentary view of one end of the electrode similar to therighthand end of Fig. 2, but showing the electrode after being coatedwith electron emissive material;

Fig. 7 is an elevation of a glass end with an electrode; and

Fig. 8 is a fragmentary elevation of one end of a slimline fluorescenttube.

Referring to the drawing, and more particularly to Fig. l, thefluorescent lamp there shown is of the slimline type, and comprises aglass tube 12 coated on the inside with conventional phosphors orfluorescent coating. The ends have caps 14 and 16, with a singleterminal pin 18 and 20 on each end.

Referring now to Fig. 7, the electrode 22 is in helical form, and issupported by spaced, relatively rigid support wires 24 and 26 which passthrough a glass seal or reentrant stem 28 formed integrally with afrusto-conical glass end 30. The support 24 is extended as a wire 32,and the support 26 is extended as a wire 34.

The glass end shown in Fig.7 ishoused in one cap (14 or 16) of thefluorescent tube 12 shown in Fig. l, and is sealed to the glass tubearound the periphery 36. The electrode at the other end of the lamp isnearly similar, and is similarly mounted in a conical glass end, exceptthat the other glass end is provided with a glass tube (not shown)projecting outwardly from its center. This tube may be connected to asuitable vacuum apparatus to exhaust the lamp.

Referring now to Fig. 2, the helix 22 supported between wires 24 and 26may be a coiled-coil, and is preferably a coiled-coil-coil. Morespecifically, a filament wire is initially coiled to form a long slenderminor coil. The individual turns of this minor coil are shown at theright end of Fig. 2, and for the rest of the helix are indicated by thedouble line 40. The minor coil is itself coiled to form what isheretermed a major coil, and the turns or coils of the major coil areshown at 42. These are disposed on a relatively straight axis at eachend where they are held by wires 24 and 26, and are disposed on ahelical axis between the wires 24 and 26, in order to form the main orlargest coils or electrode helix 22.

Fig. 3 corresponds to a fragment of the major coil taken, for example,at the straight part 48 in Fig. 2. Fig. 3 does not show the mainelectrode helix 22 at all. Fig. 3 is drawn to larger scale, and showsthe individual turns of the minor coil. The wire itself is shown at 44.The minor coil is shown at 40, and the winding of the minor coil intothe coils of the major coil is shown at 42.

The dipping or coating of the filament preferably takes place after thefilament has been secured across the support wires 24, 26. In accordancewith my invention, the supported filament is first dipped into acolloidal suspension of graphite. Instead of dipping, it may be coatedwith the aid of a fine brush. At this time the interstices between thesuccessive coils of the minor coil are filled with the graphite, andthis change is shown by comparison of Fig. 3 with Fig. 4 of the drawing.The showing at the left of Fig. 2 also corresponds to Fig. 4, followingthe graphite coating, for the individual turns of the minor coil are notshown in most of Fig. 2.

The electrode is next either dipped in a suspension of electron emissivematerial, or it may be coated by means of a brush, and at this time theinterstices between the successive coils of the major coil are filled,as shown in Fig. 5, which may be compared withFig. 4. The appearance ofthe electrode at this time is also shown in Fig. 6, which corresponds tothe right end of Fig. 2. It is important to note that even after twocoatings, the spaces between the coils of the electrode helix are notfilled, and therefore the electrode remains a helix, as shown at 22 inFig. 7.

The electrodes are air dried after the graphite coating, and prior tothe second coating. After the second coating the electrodes are againair dried, and the glass ends, shown in Fig. 7, are then fused to apreviously coated fluorescent tube of appropriate dimension.

To drive off volatile material, and to convert the elec-' tron emissivecoating to oxides (usually initially applied as carbonates) the lamp isheated as it is evacuated. For this purpose the entire lamp is heated ina furnace while the lamp is connected to a vacuum pump, and theconductors 32 and 34 shown in Fig. 7 are connected to a heating circuitwhich passes an electrical current through the electrode filament inorder to additionally heat the same. There is a heating circuit of thistype at each end, in addition to the overall furnace heating. When theheating and evacuation of the lamp have been completed, the glass tubeleading to the pump is sealed and cut off. Because the lamp is aninstant-start lamp, the conductors 32 and 34 are twisted together toform a single conductor. Caps 14 and 16 are cemented to the ends of thelamp, with the twisted conductor 32, 34 passing through a single tubularmetal terminal pin, as shown less 20 in Fig. 8. The excess length is cutotf and the twisted wire is soldered to the terminal pin, all inaccordance with conventional practice.

The filament wire may be made of any of the metals or alloys commonlyemployed for the present purpose, most typically tungsten. The carbonapplied to the electrode is preferably a suspensionof colloidal graphitein distilled water. A paste of colloidal graphite known commercially asAqu-adag may be suspended in distilled water, using a ratio of one poundof Aquadag to anywhere from one quart to one gallon of distilled water.The density of the suspension depends on the dimensions of the coilstructure of the electrode and on the method of applying the graphite,etc.

After air drying the electrode it is next coated with the electronemissive material, which preferably comprises a mixture of barium andstrontium salts, preferably carbonates suspended in a suitable vehicle.Butyl acetate with a slight amount of lacquer added is a satisfactoryvehicle. The carbonates are changed to oxides during subsequent heatingand evacuation.

The coating of the electrodes is preferably performed after mounting thetungsten wire coils on the glass ends. Laterthe glass ends are securedto the glass fluorescent tube, and then heating and evacuation proceeds.The coated tube may be of conventional manufacture. The lamp is heatedin a furnace to a temperature of, say, 700 to 800 F. The filaments arepreferably simultaneously heated more directly by passing a suitablecurrent through the electrodes to bring them up to a yellow to whiteheat, say 900 to 1200 C., while evacuating the glass tube through aglass stem provided in one of the two glass ends. This process may go onfor, say, fifteen or twenty minutes, until a desired vacuum is reached,say less than one-half micron of pressure. The temperature is keptrelatively low, as indicated, because I do not desire to form any metalcarbides by combination of the filament with the graphite. I am notcertain of the action, and the invention may be considered largelyempirical. I believe the graphite is left unchanged, and that thealkaline earth carbonates are changed to oxides by driving elf carbondioxide gas, which is evacuated by thevacuum pump.

It is know that the useful life of the lamp, judged particularly bydarkening of the lamp surface, depends predominantly on the number ofstarts, rather than on the number of hours of continuous operation. Onetest of fluorescent lamps is to operate the same for three hours on andthree hours off, in repeated cycles. I have employed an accelerated testin which the lamps are turned on for five minutes and then otf for fiveminutes. The lamps tested were Type 96T12 (96" long and 1 /2" D) using anormal ballast current of 425 milliamperes. Ordinary lamps as previouslymade began to show appreciable darkening after two or three days of thisaccelerated test, whereas lamps made in accordance with my invention ranfor twenty to thirty days before appreciable darkening appeared.

It is believed that the method and structure of my improvement, as wellas the advantages thereof, will be apparent from the foregoing detaileddescription. It will be understood that while I have shown and describedan electrode the filament of which is a coiled-coil-coil, the inventionis also applicable to electrodes in which the filament is a coiled-coil.It will also be apparent that while I have shown and described myinvention in a preferred form, changes may be made without departingfrom the scope of the invention, as sought to be defined in thefollowing claims. In the claims the term minor coil applies to thesmallest diameter coil, shown in Fig. 3, and at the right of Fig. 2. Theterm major coil refers to the next larger diameter coil, shown in Fig.3, or at the left of Fig. 2. It will be understood that in Fig. 2 whatappears to be a single wire at the left is in fact the minor coil, withno attempt to show the individual turns of the minor coil. The termelectrode helix refers to the largest coils, which are not shown at allin Fig. 3, but which are shown in Figs. 2 and 6.

I claim:

1. A fluorescent lamp comprising a glass tube with fluorescent coatingand an electrode at each end, the electrodes each comprising a filamentwire coated with 'carbon and additionally coated with an electronemissive coating.

2. A fluorescent lamp comprising a glass tube with fluorescent coatingand an electrode at each end, the electrodes each comprising acoiled-coil of filament wire coated with graphite sufficient to fill theinterstices between successive coils of the minor coil, and additionallycoated with an alkaline earth electron emissive coating, the lattercoating being sufficient to fill the interstices between the successivecoils of the major coil.

3. A hot cathode instant-start slimline fluorescent lamp comprising aglass tube with fluorescent coating and an electrode at each end with asingle pin terminal, the electrodes each comprising a coiled-coil offilament wire coated with graphite, and additionally coated with anelectron emissive coating comprising barium and strontium oxides, thesaid coatings being sufiicient to fill the interstices between at leastsome of the coils.

4. A fluorescent lamp comprising a glass tube with fluorescent coatingand an electrode at each end, at least one electrode comprising acoiled-coil-coil of filament wire coated with graphite sufficient tofill the interstices between successive coils of the minor coil, but notto fill the interstices between the coils of the major coil, andadditionally coated with an electron emissive coating sufficient to fillthe interstices between the successive coils of the major coil, but notto fill'the spaces between the coils of the electrode helix.

5. An electrode for a fluorescent lamp, said electrode comprising afilament wire coated with carbon and additionally coated with anelectron emissive coating.

6. An electrode for a fluorescent lamp, said electrode comprisingafilament wire formed. into a coiled-coil, said filament wire beingcoated with graphite suflicient to fill the interstices between thesuccessive coils of the minor coil, said electrode being additionallycoated with an alkaline'earth electron emissive coating suflicient tofill the interstices between the coils of the major coil.

7. An electrode for a fluorescent lamp, said electrode comprising afilament wire formed into a coiled-coil, said filament wire being coatedwith graphite, said electrode being additionally coated with an electronemissive coating comprising barium and strontium oxides, said coatingsbeing sufficient to fill the interstices between at least some of thecoils.

8. An electrode for a fluorescent lamp, said electrode comprising afilament wire formed into a coiled-coil-coil, said filament wire beingcoated with graphite sufficient to fill the interstices between thesuccessive coils of the minor coil, but not to fill the intersticesbetween'the coils of the major coil, said electrode being additionallycoated with an electron emissive coating sufficient to fill theinterstices between the coils of the major coil, but not to fill thespaces between the coils of the electrode helix.

References Cited in the file of this patent UNITED STATES PATENTS1,961,814 Charlton June 5, 1934 2,479,193 Zabel Aug. 16, 1949 2,496,065O Hearn Jan. 31, 1950

1. A FLUORESCENT LAMP COMPRISING A GLASS TUBE WITH FLUORESCENT COATINGAND AN ELECTRODE AT EACH END, THE ELECTRODES EACH COMPRISING A FILAMENTWIRE COATED WITH CARBON AND ADDITIONALLY COATED WITH AN ELECTRONEMISSIVE COATING.